Semiconductor dice are normally packaged in a plastic capsule to protect them from physical damage. Connections are made to external circuitry, such as a printed circuit board (PCB), by means of metal pieces, called “lead fingers,” that protrude from the capsule and can be soldered or otherwise connected electrically. Alternatively, in a “no lead” package the metal pieces have faces that are flush with the surfaces of the capsule. Two types of “no-lead” packages are designated in the industry as QFN (Quad Flat No Lead) and DFN (Dual Flat No Lead).
A cross-sectional view of a typical “no-lead” package 10 is illustrated in FIG. 1. Package 10 contains a semiconductor die 12, a die-attach pad 14, bonding wires 16 and contacts 18, through which die 12 may be contacted electrically. Die 12 is bonded to die-attach pad 14 with an adhesive layer 20, which may be epoxy. These components are encased in a capsule 22 which is made of a plastic molding compound. Package 10 may be mounted on a printed circuit board (PCB), with the bottom surfaces of contacts 18 soldered, for example, to metal pads or traces on the PCB.
“No-lead” packages are normally manufactured by attaching a plurality of dice to corresponding die-attach pads in a leadframe. The dice and die-attach pads are then encased in a plastic molding compound, and the dice are separated by sawing to produce individual packages each containing a single die. During the sawing process, the contacts tend to separate from the plastic molding compound as a result of the torque load that is imposed by the saw blade. Also, the friction from the saw produces heat, and the differing thermal expansion coefficients of the metal contacts and the molding compound, respectively, likewise tend to cause the molding compound to separate from the contacts. Separation between the molding compound and the contacts creates paths for moisture to enter the package. This moisture may corrode the metal bonds in the package and as the temperature of the package changes cause the bonds to crack and the package to fail.
It would therefore be desirable to find a way to prevent separation between the metal contacts and molding compound during the sawing that is necessary to singulate the packages.